1. n. [Drilling Fluids]

A term used by US Environmental Protection Agency (EPA) to specify the elements cadmium (Cd) and mercury (Hg). In a broader sense, the term can be used to specify other metals for which environmental concerns exist, such as copper, lead, chromium, nickel, arsenic and zinc. NPDES permits for offshore drilling set limits on Cd and Hg concentrations in barite that go into drilling fluids to be discharged offshore. Cadmium sulfide and mercury sulfide are minerals associated with barite ores. Maximum concentrations are Cd >= 1 and Hg >= 3 ppm (mg/kg).
Reference:
Recommended Practice Standard Procedure for Laboratory Testing of Drilling Fluids, 5th ed. Washington, DC, USA: American Petroleum Institute, 1995.

See: barite,  drilling fluid,  EPA,  NPDES

2. n. [Drilling Fluids]

In general chemistry, the term refers to metals that are more dense than iron, although some texts and chemical dictionaries do not recognize this as a chemical term.